This bill bans non-scientific 'therapies' that have driven young people to depression and suicide. These practices have no basis in science or medicine and they will now be relegated to the dustbin of quackery.

Such therapies have been denounced by by mainstream psychiatry in the US. The American Psychiatric Association has condemned so-called "conversion therapy" and says it is unethical.

Earlier this year, one leading figure in the "ex-gay" movement renounced the theory that sexual orientation could be changed by therapy. Alan Chambers, president of Exodus, one of the leading proponents of reparative therapy, said there could be no cure for homosexuality.

But the idea that homosexuality can be cured – by therapy and/or prayer – persists in conservative Christian circles.

The California move has been welcomed by gay rights campaigners. Kate Kendell, executive director of the National Center for Lesbian Rights, said in a statement:

Governor Brown has sent a powerful message of affirmation and support to LGBT youth and their families. This law will ensure that state-licensed therapists can no longer abuse their power to harm LGBT youth and propagate the dangerous and deadly lie that sexual orientation is an illness or disorder that can be 'cured.'